1. Field of the Invention
The present invention relates to a process for producing a photovoltaic element. More particularly, the present invention relates to a process for producing a photovoltaic element having an improved electrode structure.
2. Related Background Art
Solar cells in which photovoltaic elements are used are attracting attention because they are able to replace conventional electric power generation such as thermal and hydraulic power generation.
There are known a variety of solar cells, such crystalline series solar cells, amorphous series solar cells, and compound semiconductor series solar cells, which are under development or used in practice. Of these solar cells, an amorphous silicon solar cell has many advantages over a crystalline silicon solar cell despite the former is inferior to the latter in terms of the photoelectric conversion efficiency. That is, the amorphous silicon solar cell has a high light absorption coefficient, it works in the form of thin film, and it can be readily made have a large area. Therefore, it is the most promising type of solar cell.
As known well, the amorphous silicon solar cell is constructed of an electrically conductive substrate of stainless steel or the like and layers of back electrode, semiconductor, and light-receiving electrode which are sequentially formed on the substrate. The light-receiving electrode is formed of a transparent conductive oxide.
On the surface of the light-receiving electrode is arranged a collecting electrode comprising fine metal wires for collecting electric current. Being arranged on the light incident side, the collecting electrode casts a shadow on the light receiving face, thereby reducing the active area that contributes to power generation by the solar cell. For this reason, it is a common practice to make the current collecting electrode to be in a thin comb-shaped form. Therefore, the collecting electrode is necessary to be formed by a material with a low electrical resistance such that it has a thin, long form and a cross section which reduces electrical resistance.
Moreover, on the collecting electrode, a so-called bus-bar electrode is formed in order to collect current which is collected by the collecting electrode. The bus-bar electrode is formed of a metal which is thicker than the collecting electrode.
As an example of such an electrode, Japanese Laid-open Patent application No. Hei 8-236796 discloses a collecting electrode formed using metal wires. FIGS. 6(a) and 6(b) are schematic views illustrating an example of the structure of said collecting electrode. Particularly, FIG. 6(a) is a schematic plan view of a photovoltaic element having said collecting electrode, and FIG. 6(b) is a schematic cross-sectional view taken along the line VI-VIxe2x80x2 in FIG. 6(a).
In FIGS. 6(a) and 6(b), reference numeral 601 indicates a photovoltaic element comprising a back electrode layer, a semiconductor layer, and a transparent electrode layer sequentially formed on a substrate of stainless steel. Reference numeral 602 indicates an etching line along which the transparent electrode layer is removed so as to prevent the photovoltaic element from being short-circuited at its edge. The etching line 602 surrounds an active area of the photovoltaic element which contributes to power generation. Reference numeral 603 indicates an insulating material 603 and reference numeral 604 a collecting electrode. The collecting electrode 604 comprises a metal wire 605 (50 to 300 xcexcm in diameter) coated with an electrically conductive paste 606 or the like, which is press-bonded to the transparent electrode layer. The electrically conductive paste has a resistivity of 10xe2x88x921 to 102 xcexa9cm so that it does not cause short-circuiting (which decreases output) in case of direct contact with pinholes in the surface of the photovoltaic element and it prevents metal migration. Reference numeral 607 indicates a bus-bar electrode for additional current collection, which serves to collect current collected by the collecting electrode 604 and output it outside the photovoltaic element.
The conventional solar cell having such structure as above mentioned has a photoelectric conversion efficiency of 8 to 10% in practice. There has been a remarkable improvement in the photoelectric conversion efficiency for solar cells. Particularly, there recently has developed a semiconductor film having an improvement in terms of the short-circuit current (Isc) and having a photoelectric conversion efficiency of more than 10%.
However, in the case where the photoelectric conversion efficiency and the quantity of current is increased, there is a problem such that the loss of generated electric power at the electrode through which current flows increases in proportion to the square of the quantity of current. In other words, even when a high-efficiency semiconductor should have been developed, there is a tendency in that the practical photoelectric conversion efficiency is considerably lowered on account of the loss that occurs at a high-resistance part when generated current is led to the external circuit. Therefore, it is necessary for the solar cell to consider an adequate current collecting type in accordance with the quantity of current generated.
The collecting electrode disclosed in the above-mentioned Japanese Laid-open Patent Application No. Hei 8-236796 is constructed such that the junction of the bus-bar electrode and the wire electrode is formed with a carbon paste (which has a comparatively high resistivity) and hence has a high resistance. The solar cell with such a collecting electrode does not secure the desired photoelectric conversion efficiency because the resistance loss at the junction increases as the quantity of current increases.
One possible way to address this problem is to form the junction of the bus-bar electrode and the wire electrode with an electrically conductive paste or the like having a low resistance, thereby reducing the resistance loss.
An example of the photovoltaic element based on such an idea is schematically shown in FIGS. 7(a) and 7(b). FIG. 7(a) is a schematic plan view of the photovoltaic element and FIG. 7(b) is a schematic cross-sectional view taken along the line VII-VIIxe2x80x2 in FIG. 7(a).
The configuration of the photovoltaic element shown in FIGS. 7(a) and 7(b) differs from that of the photovoltaic element shown in FIGS. 6(a) and 6(b) in that the carbon paste at the junction of the metal wire 605 and the bus-bar electrode 607 is replaced by an electrically conductive paste 701 (such as silver paste) having a low resistance. The silver paste whose resistivity is about one-thousandth of that of carbon paste greatly reduces the resistance loss and permits the photovoltaic element to have a desired photoelectric conversion efficiency.
The photovoltaic element shown in FIGS. 7(a) and 7(b) is liable to have such problems as will be described below with reference to FIGS. 8(a) and 8(b).
FIG. 8(a) shows an appearance of the electrically conductive paste 701 with low-resistance which has been xe2x80x9cdottedxe2x80x9d. FIG. 8(b) shows an appearance of the electrically conductive paste 701 which has been pressed and heat-cured under the metal bus-bar.
Any known dispenser may be used to make a circular dot of silver paste as shown in FIG. 8(a). The round dot is pressed and heat-cured under the metal bus-bar 607 as shown in FIG. 8(b).
The problem with dotting a electrically conductive paste on a metal wire is that the electrically conductive paste flows out along the metal wire when it is heated under pressure as shown in FIG. 8(b). Eventually, the electrically conductive paste is forced out from the metal bus-bar 607.
The electrically conductive paste which has been forced out from the metal bus-bar poses the following problems.
(1) The electrically conductive paste enters the active area surrounded by the etching line 602. It may come into direct contact with pinholes in the surface of the photovoltaic element, causing short-circuiting and impairing the original photoelectric conversion efficiency. Even if this does not happen at first, the electrically conductive paste is subject to metal migration caused by metal filler of the electrically conductive paste after prolonged use and hence is liable to cause short-circuiting. This is true particularly in the case of a thin film solar cell in which the semiconductor film is very thin.
(2) The electrically conductive paste may contaminate the pressing surface of an apparatus for pressing at the time of pressing, and the silver paste which is adhered to the pressing surface needs to be cleansed off after each pressing operation.
One way to prevent the electrically conductive paste from being forced out is to simply reduce the diameter of the silver paste 701 to be applied as shown in FIG. 8(a). This purpose can be achieved by using a dispenser equipped with a round nozzle having a smaller diameter.
However, when the diameter of the applied silver paste is diminished, a problem is liable to entail such that it is difficult to hit the metal wire with the silver paste. In actual operation, dotting is accomplished by means of a dispenser robot which delivers the silver paste at prescribed intervals. Unavoidable errors may creep depending on the positioning accuracy of the wires, elements, and application points. The smaller the dot diameter, the lower the probability that the dot of silver paste hits the wire. There may be an instance in which the silver paste misses the wire. The result is that the metal wire is not completely connected to the metal bus-bar with the electrically conductive paste. This incomplete connection may be detrimental to a desired photoelectric conversion efficiency.
The present invention has been accomplished in order to address the above-mentioned problems which is unique to utilization of the metal wires.
It is an object of the present invention to provide a method of applying a electrically conductive paste stably regardless of the accuracy of wire positioning without the electrically conductive paste being forced out from the metal bus-bar The method of the present invention contributes to the production of a stable photovoltaic element.
After thorough investigation on the solution of the above-mentioned problems, the present inventors found an optimum process for producing a photovoltaic element.
According to an aspect of the present invention, there is provided a process for producing a photovoltaic element having a collecting electrode comprising a metal wire arranged on the surface of the photovoltaic element and a metal bus-bar, the collecting electrode being connected to the metal bus-bars with an electrically conductive paste. The process comprises the steps of dotting an electrically conductive paste onto the collecting electrode such that a dotted electrically conductive paste has an elliptical form whose major axis and minor axis are respectively perpendicular to and parallel to a lengthwise direction of the metal wire as the collecting electrode, placing the metal bus-bar on the electrically conductive paste, and pressing with heat the metal bus-bar, thereby curing the electrically conductive paste.
According to another aspect of the present invention, the step of dotting is carried out by delivering the electrically conductive paste from an elliptical nozzle.
According to still another aspect of the present invention, the step of dotting is carried out by delivering the electrically conductive paste from the nozzle while moving the nozzle relatively to the photovoltaic element.
According to yet another aspect of the present invention, the collecting electrode comprises a metal wire covered with an electrically conductive coating layer.
According to a further aspect of the present invention, the step of dotting is preceded by a step of removing the electrically conductive coating layer at a prescribed portion thereof where the electrically conductive paste is dotted.
The present invention includes an embodiment in that the electrically conductive paste is composed of a polymer resin and electrically conductive particles.
In the production process to which the present invention is applied, an elliptical dot of the electrically conductive paste has a major axis in the direction perpendicular to the metal wire. Consequently, the electrically conductive past can be applied accurately regardless of the positioning of the metal wire. Hence, the production of the photovoltaic elements with improved stability can be realized. Further, the electrically conductive paste has a minor axis in the direction parallel to the metal wire. Consequently, it is possible to prevent the electrically conductive paste from being forced out from the metal bus-bar. Hence, initial properties and qualities of the photovoltaic element can be improved.
Furthermore, in order to realize the elliptical shape of the dot mentioned above, an elliptical nozzle is utilized. Consequently, dotting can be carried out considerably fast with improved productivity and with an amount of electrically conductive paste to be applied being controlled.